i get this error Blueprint Runtime Error: "Accessed None trying to read property Main HudRef". Node: Update Ammo Text Graph: Shoot Function Function: Shoot Function Blueprint: BP_FirstPersonCharacter

After you create the widget, you need to store it in the MainHudRef Variable.
eg:

Here I’m using one called Main overlay, but is the same in your case, just replace with with MainHudRef

Is kinda hard to see with the small images, but it seems you’re creating a new main hud every time you shoot?
Just access the variable, the hud should already be created just one time at the start.